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Opaque return types
Learn opaque return types in Swift through a focused explanation, a dedicated topic-specific example, an engineering boundary, and a mastery check.
opaque return types is an atomic Swift topic inside Generics. The example uses the named construct directly so you can see its syntax, data flow, and observable behavior.
Read the code by locating the exact feature named opaque return types, then trace the value before and after that feature runs. This lesson is intentionally isolated from neighboring topics: changing identifiers alone would not reproduce the same behavior because the construct itself is different.
Topic-specific example
func numbers()->some Collection<Int>{[1,2,3]};print(numbers().count)For opaque return types, test the boundary that makes the feature meaningful: invalid input, an empty value, a missing branch, lifecycle/cleanup, or a type/runtime constraint. Do not infer correctness only from the happy path.